1. Field of the Invention
The present invention relates to the preparation of compositions of matter based on mixed zirconium and cerium oxides having improved specific surface areas, in particular having high and thermally stable specific surfaces.
This invention also relates to the use of such compositions, notably in the field of catalysis, whether as catalysts, per se, and/or as catalyst supports, e.g., for the conversion of vehicular exhaust gases.
2. Description of the Prior Art
Zirconium oxide and cerium oxide are well known compounds that are particularly useful and advantageous constituents, either alone or in combination, in a wide variety of catalyst compositions, e.g., multifunctional catalyst compositions, especially catalysts suited for the treatment or conversion of exhaust gases emanating from internal combustion engines. By "multifunctional" is intended a catalyst capable of effecting not only oxidation, in particular of carbon monoxide and of hydrocarbons present in exhaust gases, but also reduction, in particular of nitrogen oxides also present in these gases ("three-way" catalysts).
It will be appreciated that such catalysts, both at the level of their compositional nature, as well as principle of action, are widely described in the literature, both patent and otherwise.
Given that the scientific theories which to date have been advanced to explain this fact still appear somewhat uncertain, and at times even contradictory, it nevertheless now appears well established that the "three-way" industrial catalysts which contain both zirconium oxide and cerium oxide are overall more efficient than those catalysts which are either totally devoid of the aforesaid two oxides, or devoid of only one of them.
In catalysts such as those indicated above, the zirconium oxide and cerium oxide, which moreover can exert a specific catalytic function and/or a simple support function for other catalytic elements such as platinum, rhodium and other precious metals, are generally present in an uncombined form, namely, these two constituents are present in the final catalyst in the form of a simple physical admixture of highly distinctive oxide particles. This results in part from the fact that these catalysts based on zirconium oxide and cerium oxide are characteristically produced by intimate mixing of the corresponding oxide powders, or else of precursors which are thermally decomposable into said oxides.
However, for a variety of reasons, an increasingly marked tendency is developing in this art to introduce and to employ the elements zirconium and cerium in the composition of the catalyst, not in a separate and uncombined form, but, to the contrary, directly in the form of true mixed oxides ZrO.sub.2 /CeO.sub.2 of essentially, and preferably even entirely, solid solution type.
Nonetheless, in such a case, and this is conventional in the field of catalysis, mixed oxides are required having a specific surface area which is the highest possible and also, preferably, thermally stable. Indeed, taking account of the fact that the efficiency of a catalyst is generally all the greater when the surface area of contact between the catalyst (catalytically active phase) and the reactants is high, it is expedient that the catalysts, both while fresh and after prolonged use at more or less elevated temperatures, be maintained in a state which is the most finely divided possible, i.e., the solid particles, or crystallites, comprising same should remain as small and as individualized as possible. This cannot be attained except by starting from mixed oxides having high specific surface areas and which are relatively stable to temperature.